Death Throes. When the balrog dies, if it's Fire Aura has not been extinguished, it explodes, and each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 70 (20d6) fire damage on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in that area that aren’t being worn or carried, and it destroys the balrog's weapons.

Fire Aura. At the start of each of the balrog’s turns, each creature within 5 feet of it takes 10 (3d6) fire damage, and flammable objects in the aura that aren’t being worn or carried ignite. A creature that touches the balrog or hits it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.

Magic Resistance. The balrog has advantage on saving throws against spells and other magical effects.

Magic Weapons. The balrog's weapon attacks are magical.

Water Weakness. If at least three quarters of the balrog is submerged, it looses the Fire Aura trait. It regains the trait once it finishes a long rest

Multiattack. The balrog makes two attacks: one with its flame sword and one with its flame whip.

Flame Sword. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) slashing damage plus 13 (3d8) fire damage. If the balrog scores a critical hit, it rolls damage dice three times, instead of twice.

Flame Whip. Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 15 (2d6 + 8) slashing damage plus 10 (3d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward the balrog.

The balrog can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The balrog regains spent legendary actions at the start of its turn.

Attack. The balrog makes a flame sword or flame whip attack.

Move. The balrog moves up to it's speed without provoking opportunity attacks.